Inelastic strain rate in the seismogenic layer of Kyushu Island, Japan

Satoshi Matsumoto,Takuya Nishimura,Takahiro Ohkura

doi:10.1186/s40623-016-0584-0

Satoshi Matsumoto, Takuya Nishimura + Show 1 more

Open Access

https://doi.org/10.1186/s40623-016-0584-0

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Abstract

Seismic activity is associated with crustal stress relaxation, creating inelastic strain in a medium due to faulting. Inelastic strain affects the stress field around a weak body and causes stress concentration around the body, because the body itself has already released stress. Therefore, the understanding of inelastic deformation is important as it generates earthquakes. We investigated average inelastic strain in a spatial bin of Kyushu Island, Japan, and obtained the inelastic strain rate distribution associated with crustal earthquakes, based on the analysis of fault plane solutions and seismic moments. Large inelastic strains (>10−7 year−1) were found in the Beppu–Shimabara area, located in the center of Kyushu Island. The strain rate tensor was similar to that of the stress tensor except the absolute value in the area, implying that the inelastic strain was controlled by the stress field. The 2016 Kumamoto earthquake sequence (maximum magnitude 7.3) occurred in the Beppu–Shimabara area, with the major earthquakes located around the high inelastic strain rate area. Inelastic strain in the volume released the stress. In addition, the inelastic strain created an increment of stress around the volume. This indicates that the spatial heterogeneity of inelastic strain might concentrate stress.

Highlights

Stress and strain fields in the crust are important parameters for understanding earthquake activity and tectonic formation
We have demonstrated large inelastic strain distributed around the B–U area
Nishimura and Hashimoto (2006) and Wallace et al (2009) suggested that a shear zone, which might cause aseismic behavior, is required around the southern edge of the B–U area in Kyushu in order to explain the surface deformation detected by global navigation satellite system (GNSS)

Summary

Background

Stress and strain fields in the crust are important parameters for understanding earthquake activity and tectonic formation. The inelastic deformation associated with small earthquakes in this study is not always located along fault planes but is distributed in a volume This suggests that the deformation of the seismogenic zone occurs along active faults and in other regions. One of the principal stress axes around the shear zone is oriented normal to it (Matsumoto et al 2015), suggesting the shear zone behaves as a weak fault This is a possible explanation of the declining σHmax, resulting in a uniaxial inelastic extension

Findings

Discussion

Conclusion